1. Field of the Invention
This invention relates to a thermoplastic polyurethane resin, more particularly to a thermoplastic polyurethane resin improved in hydrolysis resistance, heat deterioration resistance, temperature dependency and compression set.
2. Related Background Art
Thermoplastic polyurethane resins have excellent physical properties such as high tensile strength, high fatigue resistance, good low temperature flexibility, etc., and also good abrasion resistance and therefore have very excellent characteristics as compared with other thermoplastic resins.
Further, thermoplastic polyurethane resins are suitable for the production of articles of small size such as precise parts including packing, sound-damping gear, bearing, joint, parts for precise machines, automative parts, electronic instrument parts, etc. Also, they can be molded into belt, hose, tube, sheet, film, etc. by extrusion.
However, when compared with cast molded elastomers among urethane rubbers, the thermoplastic polyurethane resin is inferior in compression set, and particularly when employed for packings, deformation becomes greater to be inferior in sealing characteristic, whereby liquid leak, air leak, etc. may be caused to be lowered in performance as packing to cause a problem. Also, due to presence of temperature dependency, it has the drawbacks that hardness change is liable to occur, etc.
The temperature limit at which general thermoplastic polyurethane resins of the diphenylmethane diisocyanate (hereinafter abbreviated as MDI) type can be used is about 100.degree. C. and for this reason, it has been desired to improve tubes, coatings, packings, etc. to be used around engines of automobiles under elevated temperatures atmosphere in aspect of heat resistance.
However, said polyurethane resins have excellent performances as compared with other thermoplastic resins (e.g. polyvinyl chloride type, polyester type, polystyrene type, etc.) concerning oil resistance, abrasion resistance, etc., and therefore when used as automobile parts, materials with little hardness change even at elevated temperatures have been desired. Among said thermoplastic resins which can be used under relatively higher temperatures, there is a polyester elastomer, but it is inferior in aspect of compression set, abrasion resistance, etc. as compared with thermoplastic polyurethane resins.
On the other hand, concerning thermoplastic polyurethane resins (hereinafter abbreviated as TPU), it has been known to improve hydrolysis resistance, heat resistance, etc. by use of a polycarbonate polyol, but most of these TPU use MDI as a diisocyanate component, and these MDI based polycarbonate type TPU had various drawbacks such as inferior low temperature characteristic, great temperature dependency of hardness, inferior compression set, poor impact resilience, etc., and it has been demanded to improve such drawbacks.